Bidirectional pure sine wave inverter

A project of mine requires a bi-directional inverter that will take a 12VDC (most likely) battery to single phase 120VAC pure sine wave for utility tie (mains power). I will also require the inverter to be able to rectify to utility's power to charge the battery as well.

Since I am in my final year of electrical engineering, I am more familiar with the theory and implementation rather than the products and the market of inverters.

What I do not understand too well are the inverters that are labeled as bi-directional.

That is, an inverter designed to function as a charger for the battery. What I dont understand is that for a 1000-1500W bi-directional inverter, the cost of such product will find itself in the range of $1500-2000. In comparison, a regular pure sine wave inverter of same size will cost at least $500.

My question is, where does this additional cost of $1000 come from? What aspect or portion of the inverter has been designed to be bi-directional? I understand battery management features such as 3 stage chargers.. Would a "regular" $500 inverter be able to behave as a rectifier (with the assumption that a full bridge topology is used?)

Originally, I had intended to buy components to the level of mosfets, drivers, analog filters, etc. but I do not believe I will have the time to put it together (in approx. 4 months). Since I need such an inverter for my project, but only say... $300-400 budget, can the additional features that a truly bi-directional inverter has be added to a regular inverter ? What are my options?

Sorry but as you can tell this is my first power project, and any enlightment would be greatly appreciated